<template>
  <div>
    <ul class="aside">
      <li
        v-for="item in letters"
        :key="item"
        :ref="item"
        @click="handleCityClick"
        @touchstart="handleTouchStart"
        @touchmove="handleTouchMove"
        @touchend="handleTouchEnd"
      >{{item}}</li>
    </ul>
  </div>
</template>

<script>
  export default {
    name: 'Aside',
    props: {
      cities: Object
    },
    data () {
      return {
        touchStatus: false, // 开关
        timer: null,
        startY: 0
      }
    },
    computed: {// 计算属性
      letters () {
        const letters = []
        for (let i in this.cities) { // 将cities转为数组
          letters.push(i)
        }
        return letters
      }
    },
    updated () {
      this.startY = this.$refs['A'][0].offsetTop
    },
    methods: {
      handleCityClick (e) {
        //  console.log(e.target.innerText)
        this.$emit('change', e.target.innerText)
      },
      handleTouchStart () {
        this.touchStatus = true
      },
      handleTouchMove (e) {
        if (this.touchStatus) {
          // 性能优化 函数截流
          if (this.timer) {
            clearTimeout(this.timer)
          }
          this.timer = setTimeout(() => { // 一次性定时器
            const touchY = e.touches[0].clientY - 87
            const index = Math.floor((touchY - this.startY) / 20)
            // console.log(index)
            // 向父级触发事件
            if (index >= 0 && index < this.letters.length) {
              this.$emit('change', this.letters[index])
            }
          }, 20)
        }
      },
      handleTouchEnd () {
        this.touchStatus = false
      }
    }
  }
</script>

<style lang="stylus" scoped>
  .aside{
    width:.2rem;
    position :absolute;
    top:.8rem;
    right:0;
    bottom:0;
    display:flex;
    flex-direction column
    justify-content center
    li{
      text-align center
      line-height .2rem
      color:#00bcd4;
    }
  }
</style>
